Multiplex signaling system



Filed June 19 H. NYQUIS'? I H n E 5 West i I m 2 M! l/ INVIzNIOR V 17 Jails-1i BY iya ATTORNEY Patented Aug. 31, 1926. v

UNITED STATES. PATENT OFFICE.

HARRY NYQUIST, OF JACKSON HEIGHTS, NEW YORK, ASSIGNOR TO AMERICAN TELE-PHONE AND TELEGRAPH COMPANY, A CORPORATION OF YORK.

nun'rrrnnx SIGNALING SYSTEM.

Application filed June 19, 1924. Serial No. 721,124.

An object of my invention is to provide a new and improved method andsuitable apparatus for utilizing a line for the simultaneoustransmission of a number of messages. Another ob]ect of my invention 1sto provide for a system of multiplex telegraphy with discrimination bydifference of phase between messages in the same frequency range. Stillanother object of my invention relates to providing for magnitudediscrimination as well as for phase discrimination between differentmessages transmitted within the same frequency range. Still 811- otherobject of my invention is to provide suitable apparatus and method forreceiving messages superposed on a carrier current and depending onphase discrimination or magnitude discrimination, or both, todistinguish them. These and various other objects of my invention willbecome apparent on consideration of a specific example of practiceaccording to the invention, which I have chosen to illustrate anddescribe in this specification. With the understanding that thefollowing disclosure relates to this particular example of the inventionand thatthe scope of the invention will be indicated in the appendedclaims, I now proceed to describethe structure and mode of operation ofthe apparatus shown in the drawings.

Figure 1 is a diagram of multiplex carrier current sending equipment,and Fig 2 is a diagram of the corresponding receiving equipment.

At the station West shown in Fig. l, the constant speed motor M drivesthe inductor alternators, of which three are shown, G G and G,,' ofdifferent carrier current frequencies. The output current from thegenerator G, goes over the circuit 21-22- 23-24 through an inductancecoil of impedance +jn, a resistance n and a condenser of impedance y'nin series, where n.

.has any suitable value, say 600 ohms. The value it is the.resistance ofthe generator'G, looking into it across the points 21, 24 and it isimportant in the particular example illustrated that the impedance ofthe generator should be substantially a pure resistance as seen from itsoutput terminals. The network 21-22--23-24t is a balanced Wheatstonebridge because the product'of the impedance of one pair of oppositearms.

is equal to the corresponding product for G, will lead by 4-5 degrees.

the other pair of opposite arms. Hence there is no interference in thetransformer winding '37 due to electromotive force in the winding 38,nor vice versa. This istrue for the particular frequency of thegenerator G It is also true for other frequencies because with avariation of frequency the impedance of the coil and the impedance ofthe condenser will vary inversely and, therefore, their product will beconstant, and equal to (-I-jn) (-y'n) =n Looking across the circuit at25 in the direction of the arrow, the impedance is 'n; this is truebecause the circuit of the coil 38 may be considered open on account ofthe bridge balance, and it followsthat the impedance between the points21 and 23 is i i +1 )+(n-i The currents in the'windings 37 and 38 are 90degrees apart in phase. .To prove this, notice that if a given voltageis impressed across the points 21 and 23, the current into the generatorwill lead by -15 degrees. The design is such that the impedance acrossthe terminals of the coil 37 is effectively a pure resistance. Thisbeing'the case, it follows that the current in the winding 37 'due toavoltage from the generator Similarly, it can be shown that the currentdue to the generator Gr in the winding 38 will lag 45 degrees, and hencethe currents in .the windings 37 and 38 are 90 degrees apart.

To explain the operation of the keys K and the resistances 31 and 32 areremoved and that the points 27 and 39 are connected by a resistanceequal to the' resistance 30. The points 28 and 39 will then be neutralpoints, no matter what current flows in the winding 37, and there willbe no transmission. It will readily be seen that while the key K remainsclosed, the key K, gives zero resistance from 27 and 39 when it isclosed for marking and infinite resistance when it is open for spacing.As is well known, when two unequal resistances are joined, it ispermissible and often convenient to look at the point of junction as apoint of reflection and to consider that. there is a reflected currentwave at the point of junction. In the case here under consideration, the

' and K in Fig. 1, first assume that the keys.

reflected wave from point 27 is transmitted from the network N throughthe filter F in part into the line. It is also well known that in thelimiting case when one of the resistances is made zero and the other isleft finite, the inflected current is equal to and in phase with theincident current, whereas in the limiting case when one of theresistances is made infinite and the other is made finite, the reflectedcurrent is equal to and in phase opposition to the incident current. If,therefore, the impedance at 27 is changed from'zero to infinity, theeffect is to reverse the current transmitted to the line. Now if aresistance artificialline is connected between point 27 and the pointofreflection and if'this artificial line is so designed that there is noreflection at point 27, thenthe incident wave will traverse the networkand be attenuated; moreover, the reflected wave will traverse the linealso. The net result of the artificial line is that the currenttransmitted to the transmission line is attenuated by twice the loss ofthe artificial line. \Vhile the function of the key K is to open andclose the circuit and thus to reverse the phase 6f the transmittedcurrent, the function of key K is to add or remove artificial line andthus to change the magnitude without changing the phase.

Thus it will be seen that the key K transmits by phase reversal of thecarrier current and the key K transmits by changingthe magnitude of onecomponent of the carrier current from generator G In a similar manner,the keys K and K respectively, transmit by phase-reversal and magnitudechange in the other carrier current component which differs 90 de ees inphase from that controlled by the eys K and K These two carrier currentcomponents of the same frequency, difl'ering 90 degrees in phase andvarying in magnitude as determined by the operation of the keys K, and Kare superposed in the input of the band filter F, whose output oes inmultiple with the outputs from oter similar filters to the line 1 Thesefilters are confluent band filters each with relatively narrow bandwidth and with the carrier frequency near the middle of its range so asto produce no serious phase shlft during the building up of the signals.

The box S, represents a network similar to that between the generator Gand the filter F Accordingly, four more messages will be carried in thechannel from the generator Gr through the network S and the filter F,and added inmultiple on the line Z 1 Alternating current of stillanother frequency from the generator G is passed through the artificialline or network N, by which it is reduced to the proper intensity to beput on the line. The simple unmodulated current of desired intensitygoes ,SM, whic these, identified respectively with the generators G andG each carry four messages. The frequency in the third channel,identified with the -generator G may be called a pilot frequency and thechannel may called a pilot channel. As will be pointed out presently,this channel is utilized to provide "for synchronism at the receivingend.

Referring to Fig. 2, this shows, the receiving station which may becalled East. The currents comin in over the line Z, pass through the ajustable artificial line or network N, which is controlled automaticallyin a manner to be described presently. This net work N' attenuates thecurrentsv so as to give a uniform 'm itude in. its output for the samesignal e ements from one time to another. The currents,

thus attenuated to a standard magnitude,

pass through the amplifier or terminal repeater A to the band filtersF',, F and .F, in multiple. Thesefilters correspond in ,their frequencyranges, respectively, with the filters and F, at station West.

The current of through the-filter F 1 to the amplifier A, and thence toa power tube amplifier A, whose out ut goes to the synchronous motordrives the two generators Gr and Gr of the same frequency, respectively,as Gr and G at station West.

The voltmeter relay V is connected across the output from the amplifierA and normally holds its index at a certain position corresponding to acertain voltage. Whenever this voltage changes .a little either way, theindex closes one or. the other of the circuits'for themagnets 44 and.45, causing the armature 43 to move and bring the wheel 41 in contactwith one side or-the other of the yoke 42. The wheel 41 is rotatedcontinuously by the motor M so that the artificial line N is adjustedone way or the other to adjust the attenuation there through. Thus itwill be seen that by means of the voltmeter relay V and the associatedapparatus, the current of pilot frequency is kept at uniform voltage asdelivered to the input of the amplifier A,. This insures that for thesame signal elements in the other channels, the voltages delivered tothe input of the amplifier A will be uniform. Even if the attenuation onthe line Z varies from time to time. as may be the case due to weatherconditions, etc., there will be no corresponding variation of thecurrent and voltage magnitudes in the output from the network N atstation East. If the received currents at East .were to vary from timeto time, it might happen that sigpilot frequency goes nals of lowmagnitude would sometimes be A strong enough to affect the low magnitudereceiving apparatusor that'high ma itude would sometimes be too weak toa cot the. high magnitude receiving apparatus. By the adjustment of .N'any such outcome is prevented."

The modulated current of carrier frequency determined by the generatorGr at station West will accordingly find its way through thefilter F 8to thewindings 50 of the alternating current relays" to be describedpresently.

There are four of these relays, PR MR PB and MR Each of them is of thedynamometer type comprising two members each energized by alternatingcurrent and relatively movable. In the relay MR the winding for onemember is 50 and the wind in for the other member comprises the twocoils 57 and 58. As will readil be seen, the

winding 50 carries the receive carrier current of a certain frequencycomprising two phase components each modulated 'byphase reversals andby-magnitude changes.

The current in the coils 57 and 58 is generated locally. The generatorGr is of the same frequency as the carrier belonging to the band passedby the filter F The out put current from this .generator goes through anadjustable phase shifter P and then through a phase splitter J whoseprinciple is the same as that at the station West associated with thegenerator G,.

In the output circuits 5152 from the phase splitter J the currents aredegrees agart. The conductors 51 o to two opposite vertices 53 and 54 0%a Wheatstone bridge whose other two vertices 55 and 56 g are joined by aresistance. The dynamometer windings 57 and 58 of the relay MR are intwo opposite arms of the bridge and the corresponding windings of'therelay PR are in the two remaining opposite arms of the same bridge.

The bridge arrangement makes the operatlon of either relay R, or MRindependent of any reaction involved in the operation of the other ofthese relays.

The current delivered by the conductors 51 will be in phase with one ofthe two carrier components from the filter F 8 and 90 degrees from theother. The currents in phase in the windings 50 on the one hand and 57,58 on the other hand, will produce effective armature movement if ofsufficient magnitude, but the currents 90 degrees apart will .tend toproduce movements only of the carrier frequency to which the armaturewill be much too slow to respond effectively. Moreover, the relay MR ismarginal and adjusted so that in the absence of current its armaturestands midway between the two,

contacts shown and it Will only close on one or the other of thesecontacts when current is of the intensity determined b the closure ofthe key K at station est. Phase reversals as determined by the keyK atWest 1 will merely shift the armature quickly from one contact to theother as 7 Ion as the key K remains closed. The relay lulR controlsaneutral sounder N8 The relay PR is like the relay MR except that itsarmature is thrown one' way or the other by current ofany magnitude. Onone contact the armature applies positive current to the polar sounderPS and on the other contact it applies negative current. Thus it.will beseen that the armature of the rela PR responds to the operation of thekey 1 at station West independently .of the current magnitude asdetermined by the key K From the description that has gone before, itwill readily be seen how the other phase component of the receivedcarrier current operates the polar relay BR, according to 'which theselocally generated currents are applied, and conductors from oppositesides of each bridge to the remaining windings of the respectivedynamometer relays.

.2. Means to receive and discriminate signals on different phases anddifferent mrgnitudes of alternating carrier current of a certainfrequency, comprising dynamometer relays each with one winding energizedby the received current and each with another winding energized by localcurrent in phase with a desired component of the received current, onesuch relay being marginal.

3. Means to receive and discriminate signals on different phases anddifferent magnitudes of alternating carrier current of a certainfrequency, comprising dynamometer relays each with one. windingenergized by the received current and each with another windingenergized by local current in one circuit in phase with a desiredcomponent of the received current, and means to prevent resultantreaction through said circuit from one relay to another.

4. Means'to receive and discriminate sig nals on different phases anddifi'ercnt magnitudes of alternating carrier current of a certainfrequency, comprising dynamomesaid windings.

ter relays each with one Winding energized by the received current andeach with another windin energized by local current, a source for' sailocal current, and a bridge through which said source is connected to 5.Means to receive and discriminate signals on different phases anddifferent magnitudes of alternating carrier current of a certainfrequency, comprising dynamometer relays each with one winding energizedby the received current and each with another winding energized by localcurrent in phase with a desired component of the received current, onesuch relay being effectively responsible only to polarity changes andanothersuch relay beingefl'ectively responsive only to magnitudechanges.

6.'In combination, a pair of dynamometer relays, with windin s energizedwith current of one phase, an a pair of dynamometer relays with windingsenergized with currentof phase at 90, and all with windings energized.from an incoming circuit, one relay of each pair being responsive topolarity changes and the other relay of each such pair being responsiveto magnitude changes, whereby one relay of each pair responds topolarity changes in the properly related component of the incomingcurrent and not to the component at 90 thereto.

In testimony whereof, I have signed my name to this specification this18th day of HARRY N YQUIST.

June, 1924.

